I was about to post a HPGCC3 program in the software library when the forum hit me with the message that my attachment type was not allowed. The extension was HPGCC3's default .ex3.
When I tried my backup plan, which involved changing its extension to the ConnKit's preferred extension .hp, I got the same message.
Of course I could compress the file, but a ZIP archive containing a single 15KiB file is neither convenient nor particularly memory-saving.
Could these attachment types be allowed? Preferably together with other common file types for HP calculator software, e.g. .hpprgm for Prime programs.

Okay, that's a hurdle I did not expect. I have next to no knowledge about that stuff, but after reading up on it, I'd go with the generic "application/octet-stream". (At least my mail client chose that when I sent a mail with .hp and .ex3 attachments to myself.) If that is not satisfactory, a type will have to be made up and registered - while searching through the list of registered types, I couldn't find a matching type other than that generic one.